This paper investigates the impact of fiber core diameter on thin-core fiber grating sensing applications, addressing challenges such as low optical imaging resolution, high light energy loss, and poor imaging quality caused by core diameter mismatches in access fibers. Using a dual-grating quadratic dispersion demodulation optical system as a model, a comparative analysis was conducted between conventional single-mode fibers and thin-core fibers with varying core diameters. The study examined the effects on imaging and demodulation accuracy by simulating changes in imaging spot resolution, beam divergence angle, receiving surface irradiance, and the modulation transfer function (MTF) curve at the cutoff frequency. The computational results reveal that reducing the fiber core diameter from 8 μm to 2 μm improves resolution from 0. 8 nm to 0. 1 nm and increases irradiance on the receiving surface. The optical energy utilization on the receiving surface reaches its maximum of 74. 98% when the core diameter is below 4. 8 μm, and the exposure time decreases to 36% of the conventional state. Additionally, the MTF curves at the cutoff frequency show a parabolic correlation with core diameter, with images approaching the diffraction limit when the core diameter is 4. 2 μm. This study establishes and validates the correlation between fiber core diameter and spectral demodulation imaging indices, offering valuable insights and references for fiber grating sensing demodulation technologies with different core diameters.